1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser beam printer, which uses electrophotography, and more particularly, it relates to an image forming apparatus which has a plurality of image forming portions and which can record or form a colored image or a monochromatic image on a sheet such as a transfer member fed by a belt-shaped feed means.
Further, the present invention relates to an image forming apparatus such as an ink jet printer, which can record an image on a sheet fed by a belt-shaped feed means by the use of ink droplets formed by thermal energy or other appropriate means.
2. Related Background Art
In a conventional image forming apparatus of this kind, a plurality of pairs rollers have been used as a feed means for feeding a transfer member (sheet) between a plurality of image forming portions arranged side by side. It has also already known to use a belt in place of such paired rollers.
However, when the plurality of rollers are used as the feed means, there arises a drawback that a leading edge of the sheet is damaged since the leading edge of the sheet is repeatedly pinched between the paired rollers; thus, it is hard to say that the feed means of this kind is excellent since it cannot effectively attain the increase in a feeding speed of the sheet and/or the feeding stability of the sheet according to the kinds of the sheets. Further, since a length of a feedable sheet is determined by a distance of the roller pairs, a size or dimension of the available sheet will be limited.
On the other hand, as disclosed in the Japanese Patent Laid-Open Nos. 62-78577 and 62-146876, when the belt-shaped feeding member (feeding belt) is used as the feed means to feed the sheet toward a transfer position for an image forming medium and then toward a fixing means, the sheet is fed from a sheet supply entrance to the fixing means while remaining the sheet on the feeding belt. Therefore, with the feed means of this kind, there is obtained an advantage that the possibility of erroneous or poor feeding of the sheet is less occurred than that in the case of the above-mentioned roller feed means. Further, when the roller pairs are used as the feed means, since the number of portions slidably contacted between the sheet and elements of the image forming apparatus is increased, there arises a drawback that the sheet is electrically charged due to friction and/or paper powder is generated if the sheet comprises a paper. On the contrary, when the feeding belt is used as the feed means, such problems are not occurred at all.
However, in the image forming apparatus including the belt-shaped feed means, there arise problems that toner powder used to form the image is easily adhered to the feeding belt and that, in particular if the poor feeding of the transfer sheet is occurred before the transfer sheet reaches the transfer position, the image will often be transferred onto the belt since an image bearing member is contacted with the belt. And, such smudge of the belt will cause the smudge on the back of the transfer sheet, and/or the reduction in transfer efficiency while the image is being transferred from a surface of the image bearing member to the sheet, thus causing irregularity in the transfer.
Accordingly, in the conventional image forming apparatus having the belt-shaped feed means, it was necessary to provide a cleaning mechanism for removing the toner powder adhered on the surface of the feeding belt.
However, if the image formed on the surface of the image bearing member is transferred to the feeding belt, since the image is electrically charged by a transfer charger, it will be difficult to remove the transferred image from the feeding belt. In order to remove the image completely from the belt, the belt portion onto which the image is transferred must be passed through the cleaning mechanism repeatedly, thus causing a problem that the cleaning time is extended. Particularly, in the conventional image forming apparatus wherein a plurality of image forming portions were arranged side by side in confronting relation to the feeding belt to form the colored image, since the peripheral length of the feeding belt was lengthened, it took a considerably long time to clean the whole feeding belt, and, thus, there arose a problem that a normal condition (i.e., image formable condition) could not be restored for a short time if the poor feeding was occurred.